Bell peppers are the greatest fruit/vegetable. At the very least they should be in everyone's top 5.

Nutrition

One single bell pepper gives 150-300% DV for Vitamin C, which is insane. When it comes to Vitamin C everyone thinks of oranges or lemons, but both of them sit at 50-90% DV, well below bell peppers.

Bell peppers are also a decent source of Vitamin A and fiber (but not too crazy), and are low in fat & sugar.

I'm not going to claim that bell peppers are the greatest when only considering nutrition. Consider the following ranking of fruits/vegetables by nutritional value:

source

As you can see, bell peppers, although still impressive, are not too high up the list.

Taste

However, take a look at the fruits/vegetables above it on the ranking. Chicory? Kale?? "Mustard green"? Spinach????? Needless to say bell peppers are better tasting than these disgusting plants. So bell peppers strike an incredible balance between nutrition and taste. When eaten raw, they have a light sweetness and contain an insane amount of water, making them very refreshing.

Ease of eating

Granted, there are some more normal plants that ranked above bell peppers in terms of nutrition: lettuce variants, parsley, etc. But many of them are leafy (meaning trickier to wash) or simply unsuitable to be eaten raw.

Given the sleek, aerodynamic shape of a bell pepper, it is very easy to wash. What's more, every part of it is edible raw (yes, even the seeds and stem, though admittedly they are not as pleasant to eat), so you don't have to navigate around any parts of it awkwardly.

Since you can eat the whole thing raw, you can casually bite into it, as if eating an apple. Except it's a lot easier to chew than an apple too.

Flexibility

As established earlier, bell peppers are very easily eaten raw. However, they are also great stir-fried, roasted, grilled, etc.

Conclusion

Yes, there's a lot of fruits/vegetables that are better than bell peppers in nutrition, it's not the ultimate super-food. However, when you look at the ones who have better nutrition, many of them are disgusting to eat, unfit to eat raw, less simple to eat, or less tasty. Bell peppers are at the intersection of nutritious, easy to eat, and delicious. The only real drawback of bell peppers is that they are slightly more on the expensive end.

Obviously, my opinion, etc.

Fun fact

Purple bell peppers actually exist:

I've never actually seen one at a supermarket...